Picture taken April 4, 2014. REUTERS/Nick Perry/Pool A crew member aboard a Royal New Zealand Air Force (RNZAF) P3 Orion maritime search aircraft watches a smoke flare after it was deployed to mark an unidentified object while flying over the southern Indian Ocean to look for the missing Malaysian Airlines flight MH370 April 4, 2014.

SYDNEY (Reuters) - The head of the Australian agency supervising the search for a missing Malaysia Airlines plane said on Monday that an autonomous underwater vehicle would soon be deployed, moving the search underwater after nearly six weeks of fruitless searching.

Malaysia Airlines Flight MH370 disappeared soon after taking off on March 8 from Kuala Lumpur bound for Beijing with 227 passengers and 12 crew on board.

Searchers are confident they know the approximate position of wreckage of the Boeing 777, some 1,550 km (963 miles) northwest of Perth, and are moving ahead on the basis of four acoustic signals they believe are from its black box recorders.

"Despite the lack of further detections, the four signals previously acquired taken together constitute the most promising lead we have in the search for MH370," Angus Houston, the Australian search chief, told reporters in Perth.

"The experts have therefore determined that the Australian Ocean Shield will cease searching with a towed pinger locator later today and deploy the autonomous underwater vehicle, 'Bluefin-21', as soon as possible," he said, referring to the U.S. Navy device designed to detect the tell-tale "pings".

The batteries in the plane's black box are now two weeks past their 30-day expected life and searchers will be relying on sonar and cameras on the Bluefiun-21 to detect the box.

Houston added that although an oil slick was located in the search area on Sunday evening, he was pessimistic about the likelihood of finding any of the floating debris.

"The chances of any floating material being recovered have greatly diminished and it will be appropriate to confer with Australia's partners to decide the way ahead later this week," Houston said.